The invention relates to non-impact print apparatus, especially to ink jet printers capable of producing drops of ink on demand.
Drop-on-demand printing is well known in the prior art. A typical problem associated with this art is breaking the surface tension of the printing liquid. Other problems are creating drops singly, and uniformly, and controlling the placement of the drops on a record member. Various solutions to each problem have been explored in the prior art.
Prior art drop printers overcome the surface tension of the printing liquid either by applying electrical or acoustical pulses near the surface of the liquid or by creating a momentary reduction in the volume of the print liquid reservoir which squeezed a proportionate volume of the print liquid out of the orifice of the printer. U.S. Pat. No. 3,596,275 entitled "Fluid Droplet Recorder" and issued to Richard G. Sweet on July 27, 1971, uses hydrostatic pressure near the orifice to force a stream of print liquid from its nozzle. Control of the electrical or acoustical pulses or of the change in volume provides control of the volume of the drop produced. Prior art drop-on-demand ink jet printers generally produce drops of print liquid having a diameter of 1.9 times the internal diameter of the print nozzle. Therefore, the ratio of the internal diameter of the print nozzle to the diameter of the drop has traditionally been 1:2.
Although control of drop volume is necessary in controlling drop uniformity, control of volume is not sufficient to control the number of drops produced. A common problem in prior art drop printing was production of more than one drop at a time. Solutions to the multiple drop problem attempted either to produce single drops or to deflect unwanted drops away from the record number. A technique for reduction in the number of unwanted drops was described in U.S. Pat. No. 3,840,750 entitled "Pulsed Droplet Ejecting System" and issued to Steven I. Zoltan on Oct. 8, 1974. The apparatus disclosed therein reduced the diameter of the orifice to the point that surface tension prevented the print liquid from emerging when the activation mechanism was quiet. Deflection of unwanted drops away from the record member was disclosed in U.S. Pat. No. 3,298,030 entitled "Electrically Operated Character Printer" issued to Arthur M. Lewis and A. D. Brown, Jr. on Jan. 10, 1967, by electrostatically charging the emerging droplets and using electrically charged plates to steer them, and in U.S. Pat. No. 3,416,153 entitled "Ink Jet Recorder" issued to Carl H. Hertz, et al. on Nov. 26, 1968, by propelling the ink jet through an opening in a shield for intercepting unwanted drops then onto the record member. These methods of droplet control were relatively complicated and expensive.
Prior art methods for controlling placement of drops on the record member were generally variations on electrostatic charging of the drops and steering of the charged drops using additional electrical sources.